Manufacture of conductors



Patented Nov. 23, 1943 UNITED STATES PATENT OFFICE MANUFACTURE OF OONDUCTOBS Henri Elchinger, Wilmington, Del.

Serial No. 260,226.

1 Claim.

rial in the highly compact and totally dehydrated state.

The insulating material usually employed hitherto in these manufactures is ordinary magnesium oxide.

The. applicant has proved that the hitherto employed insulating materials, in particular ordinary magnesium oxide in uncompacted form, do not possess any natural dielectric strength and the breakdown voltage of the conductors they serve to iorm is of the same order of magnitude 'as if the insulating layer were replaced by air.

In fact, the breakdown voltage is, on an average, about 3,000 volts when the thickness of the insulating material is 1.5 mm. and 4,500 volts when the thickness reaches 3 mm., etc.

Now, these are precisely the breakdown voltages of conductors composed of a cylindrical core and of a likewise cylindrical outer sheath which are separated by air, the constant distance between the two armatures of the condenser thus formed being, according to cases, respectively 1.5 mm., 3 mm., etc.

It would therefore appear that, in these conductors, everything takes place as though the purpose of the insulating material employed was to define and ensure the geometrical position of the core in the transverse section oi the sheath, without increasing the dielectric strength of the conductors.

The present invention provides a very considerable improvement in the processes for manufacturing conductors of this category.

Said invention consists in imparting to the insulating material by an appropriate treatment, a property of dielectric strength which it does not naturally possess and which will hereinafter be called extrinsic dielectric strength," so that the conductor will be characterized by a higher breakdown voltage than that of air, even if the uncompacted insulating material used naturally possesses a property of dielectric strength that is only equal to that of air.

For this purpose, during its manufacturing process, the conductor will be subjected to a treatment of such a nature as to allow permaent pressures to subsist in the insulating material which substantially increase its dielectric strength.

Research work done by the applicant has shown him that the dielectric strength of a powdered mineral insulating material is, to a certain extent, proportional to the value oi the resilient pressures which may occur between the particles of the insulating material. Now, for a conductor which is in the process of manufacture and at a certain phase of drawing, such pressures vary within wide limits, according to whether the metal of the sheath is in the coldhammered state or whether on the contrary it has just been annealed.

It will in fact be understood that, everything being equal, said pressures of the insulating material are greatest when the conductor has just undergone the final drawing operation during which it has reached its final dimensions, since economy is naturally an incentive to continue said drawing operation as far as, inter alia, the drawing qualities of the metal allow.

Now, the metal of the sheath is then in an advanced if not in a maximum state of coldhammering, so that for delivering the cable to the consumer it is necessary to anneal the metal if it is desired to avoid in particular the risk of a breakage of the sheath during the bendings it will undergo when the conductor is being placed in position and installed.

By annealing the conductor, the insulating material will lose the state 0! pressure in which it was, and its extrinsic dielectric strength will disappear with it.

Now, the applicant has proved that the state of pressure of the insulating material varies to a very slight extent between the instant when a conductor which has been annealed is subjected for example to a very slight pass even without elongation of the rough shape, and the instant when, on the contrary, drawing is continued up to the limit beyond which a break would be produced.

Thus, if the sheath of a previously drawn and annealed conductor is very slightly cold-hammered or shaped, for example, by an appropriate additional pass, in such a manner as not to exceed for example 5% of the total elongation" of the metal, it is found that the resilient pressure of the insulating material has returned to very substantially its maximum value, and with it has regained its highest extrinsic dielectric strength, although the conductor can still undergo 95% of its "elongation and consequently, in practice, still leaves the conductor itself with all its mechanical properties and in particular its flexibility.

It is an easy matter in practice to ascertain the extend of said additional pass. It may be said that it can be chosen in such a manner as to "shape the sheath until the instant when the insulating material reaches the limiting pressure (defined in French Patent No. 795,277) that is to say, until the instant when a more considerable pass, however, small it may be, would cause the homothetic elongation of the core of the rough shape.

The applicant has proved, in fact, that the ratio between the dielectric strength of a conductor having a powdered insulating material which is treated as just explained and the dielectric strength of the same conductor after having undergone only annealing, is greater than 1.2.

The present invention includes within its scope the use of means which are capable of placing the powdered insulating material in a permanent state of pressure, capable of imparting to it a property of extrinsic dielectric strength and in particular the use, for the manufacture of the electric conductors in question and whatever be the process employed for carrying it out, of the method offoperation of shaping the sheath, which method is characterized by the fact that the metals of the sheath and of the core have both retained almost their entire capacity of cold hammering and even of their property of elongation, and by the further fact that the dielectric strength of the insulating material of the conductor in the latter state considered is at least 20% greater than the dielectric rigidity of the same sample of conductor aaaacav which has been subjected to an annealing treatment only.

In-a practical example, a conductor of about 5.3 mm., outside diameter of sheath and having an insulating layer of about 1.5 mm., thickness made of a mineral with no property of intrinsic dielectric rigidity, ordinary magnesium oxide for example, is characterized by a breakdown voltage of about 3,000 volts, after the last annealing operation of the series oi metallurgical treatments of the manufacture and it the process Just described is not applied. It, on the contrary, a very slight shaping is effected, for example, by a drawing operation which reduces the outside diameter of the sheath to 5.2, the breakdown voltage of the same conductor becomes higher than 4,000 volts.

or course, the invention applies whatever be the number of cores of the conductor, the metal which they and the sheath are made of and the particular refractory powdered insulating material used. Saicl invention covers, by way of new industrial products, conductors of the above mentioned category wherein the dielectric rigidity is at least 20% greater than the rigidity oi the same conductor which has undergone annealing only.

I claim:

A method for the manufacture of a cable which consists of an assembly of at least one conducting core, a metallic sheath and powdered mineral insulating material interposed between said core and sheath, which method comprises subjecting the saidassembly to successive shaping and annealing operations, the last of these operations being a shaping operation during which the cable is elongated to an extent which does not exceed 5% of the elongation which the cable can undergo without rupture, whereby the dielectric strength of the insulating material of the resultant cable is at least 20% greater than would be the case if the last operation were an annealing treatment only.

HENRI EICHINGER. 

